The present invention relates to equipment for giving greater stability to a vessel used in floating production systems which make use of mechanisms which enable the vessel to rotate about a vertical axis (turret).
With the discovery of offshore petroleum fields, it became necessary to use new techniques to exploit these production fields. Initially, use was made of structures fixed to the seabed but emergent at the surface, for housing the equipment needed for the oil-extraction operations.
These structures use rigid ascending pipes, known by specialists as xe2x80x9crisersxe2x80x9d, to link the petroleum well on the seabed to equipment on the structure, at the surface. This type of solution is recommended only for depths of water up to 300 meters since, for greater depths, this solution is expensive and takes a great deal of time to set up.
As new fields were being discovered in greater depths of water, new technologies appeared for solving the problem. One of these was to make use of floating platforms to accommodate the equipment needed for oil production.
Initially, platforms of the semi-submersible type were used for this function. Owing to their characteristic of having their floats below the sea surface, such platforms are little affected by the movement of the waves or other environmental conditions, such as wind and water currents. As a consequence, the semi-submersible platform may be kept on site by means of conventional mooring systems, basically anchoring lines formed by a combination of steel cables and chain-type lines cast down to the ocean floor at predetermined points and fixed by means of anchors.
In this solution, use is made of flexible ascending lines, or flexible risers, to link the various underwater oil wells to the platform. These flexible risers cost a great deal more than fixed risers but make the system so versatile that it is possible to begin production in a field only a short time after the discovery of the field.
Another solution which is being employed is the use of a conventional oil tanker equipped with a mechanism which enables the vessel to rotate about a vertical axis. This mechanism, known by specialists as a xe2x80x9cturretxe2x80x9d, basically comprises two parts joined by a bearing which allows one part the possibility of rotating with respect to the other. The lower part of the turret is kept fixed on site by means of conventional mooring systems and the upper part is connected to the structure of the tanker by means of the bearing. In this way, the tanker can rotate about the vertical axis of the turret. In this case, also, use is preferably made of flexible risers for connecting the various underwater oil wells to the tanker, and these flexible risers normally form a free catenary between the turret and the sea bottom.
As the two parts of the turret can move with respect to each other, the risers are connected to its lower part and use is made of a multiple rotary joint (swivel) or a reel of flexible lines which are rolled up or unrolled around the upper part of the turret (drag chain) to form a connection between the two, fixed and movable, parts; in other words the above-mentioned equipment enables a connection to be made between the risers, which are kept in a fixed position with respect to the vertical axis of the turret, and the connection lines thereof on the vessel.
Owing to the fact that tankers are more sensitive to environmental conditions than semi-submersible platforms, for reasons of directional stability, the turret is normally installed on one of the ends of the vessel (bow or stem) or, alternatively, use is made of an external turret fixed to one of these ends. The positioning of the turret at the ends gives the tanker a weather-vanning capacity, i.e. the ability to align its bow to face environmental disturbances in a favorable position.
For certain headings at an excessive angle to the wind or tide, it would be extremely difficult to anchor a tanker in the open sea, especially in the case of a large-tonnage tanker, owing to the constant variations in sea conditions. However, if it were possible to position the tanker so as to maintain it aligned with the changing direction of the environmental force (winds, currents, etc.) it would, at least in its central region, behave in a similar manner to a semi-submersible. Hence the option of positioning the turret at the ends of the tanker has been provided for.
However, placing the turret at the ends of the tanker gives rise to a serious drawback, which will now be described. The longitudinal rocking movement (pitching) of the tanker as a result of wave action, which is a movement of rotation about a transverse horizontal axis which passes close to the mid ship section of the tanker, means that the ends of the tanker undergo practically linear vertical movements. The further away the turret is from the mid ship section, the greater the vertical movements will be.
As these movements are transferred in their entirety to the connection of the risers, when use is made of tankers of great length such as xe2x80x9csupertankersxe2x80x9d whose length is close on 320 m, fairly high values of rise and fall at the turret can easily be achieved. These movements may have disastrous effects on the risers, particularly if free-catenary risers are used, since when the movement is one of descent it is possible for there to be a compression effect, which can easily damage the risers, with disastrous results.
One solution to the problem would be to transfer the turret to a point as close as possible to the mid ship section. It is not difficult to work out that, if the turret were placed in such a position nearer the mid ship section, drag on the turret would cause the tanker to tend to lie in a transverse position with respect to an environmental force. However, it is normally under such circumstances that the effects arising from such exposure to the environmental force are most strongly felt. In addition to the force arising from the transverse exposure of the hull, undesirable transverse rocking movements (rolling) are amplified, with possible catastrophic results as to the integrity of the entire production system.
Two solutions to this problem are currently known. The first of these attempts to control the directional stability of the tanker by means of additional active forces, for example, by use of tugs which continuously exert active forces to steer the vessel into a more favorable attitude relative to environmental conditions. This effect may also be obtained by using bow thrusters located on the tanker itself. Despite being fairly effective, this solution is expensive, due basically to the high consumption of fuel to keep the engines of the tug, or the bow thrusters, in permanent operation.
Another known solution is to prevent the risers having a free-catenary configuration by using intermediate buoys or floats which interrupt the continuity of the disturbance generated by the compression of the risers caused by the combination of movements to which the tanker is subject. This would prevent the direct transmission of the vertical movements to the point of contact of the risers at the seabed (touchdown point). This solution is also fairly effective, but is also expensive and requires greater care to be taken during installation and handling.
The present invention proposes a novel solution to the problem which is effective and inexpensive, as will be seen below.
The present invention relates to a passive stabilizer for providing greater stability to a vessel used in floating production systems which make use of turret mechanisms which enable the vessel to rotate about a vertical axis.
The present invention is embodied in a vessel used in floating hydrocarbon fuel production systems, that has a turret mechanism which enables the vessel to rotate about a vertical axis relative to risers therebelow, and wherein it includes at least one projecting, passive stabilizer body on the lower part of the hull of the vessel intended to increase the vessels directional stability in relation to environmental conditions.
In a first aspect the projecting body on the lower part of the hull of the vessel is in line with the longitudinal axis of symmetry of the vessel and is preferably at one of its ends.
In a second aspect of the present invention there is a pair of projecting bodies on the lower part of the hull of the vessel located on both sides of the longitudinal line of symmetry of the vessel, and again preferably at one of its ends.
The projecting bodies may be retractable and may have end plates which are mounted transversely to them.